1. Field of the Invention
The present invention relates to a method of exposure in photolithography for use in the manufacturing process of semiconductor devices, and more particularly to an exposure method with so-called image composing (or photocomposing) in which the circuit pattern area to be formed on a semiconductor wafer is divided into plural partial areas and super-imposed exposures are made with partial circuit patterns respectively corresponding to said partial areas.
2. Related Background Art
In recent photolithographic processes reduction (or equal-size) projection exposure apparatus has been widely used for exposing a wafer to the pattern of a reticle. Such apparatus is used for exposing the wafer, coated with photoresist, to the pattern of the reticle through a projection lens. Since the area covered by an exposure is smaller then the total area of the wafer, there is usually employed a step-and-repeat method in which the exposure is repeated in combination with the stepping of the wafer by a predetermined pitch.
In such apparatus, the minimum resolvable line width is determined by the wavelength of the illuminating light for exposure, the numerical aperture (N. A.) of the projection lens etc. The resolving power is improved as the wavelength of the illuminating light becomes shorter or as the numerical aperture increases, but these factors have limits in practice. An approach, therefore, for improving the resolving power is to increase the reduction projection rate, by reducing the area of projection exposure. This approach is used because of the difficulty in designing and manufacture of a projection lens with a large, numerical aperture, combined with a large projection area.
For example, in case a resolving power of 1 .mu.m is achieved over a projection area of 10.times.10 mm in 1/10 reduction projection with a projection lens of numerical aperture of 0.35, it is extremely difficult to re-design and manufacture said lens to achieve a higher resolving power by increasing the numerical aperture only. However, it is relatively easy to increase the numerical aperture if the projection area is reduced for example to 5.times.5 mm (corresponding to an image field of ca. 7.1 mm.phi.). In such case even a numerical aperture of 0.5 is achievable, and such projection lens can stably achieve a sub-micron resolving power over the entire projection area (5.times.5 mm). Experimentally there has been obtained a resolving power easily down to 0.8 .mu.m over the entire area, and even of 0.6 .mu.m under best conditions, with the conventional illuminating wavelength of g-line (436 nm). Such stable achievement of sub-micron resolving power could not be anticipated when reduction projection exposure apparatus, or so-called steppers, were introduced into commercial production.
A projection lens, of such high resolving power will result in limitation of the size of the circuit pattern (corresponding to a chip) to be formed on the wafer, due to the reduced projection area. In order to avoid such limitation there has been considered a method called image composing or photocomposing.
The exposures with such image composing or photocomposing method are essentially the same as those in the conventional step-and-repeat method, but the image alignment between the pattern in an area and the pattern in a neighboring area has to be extremely strictly controlled in the X and Y directions, because any joint error will result in defects, such as broken wirings, in all the jointing portions so that it is almost impossible to rescue the chip area.
It is therefore necessary to maintain not only the matching accuracy of the layers of finer patterns but also the two-dimensional joint accuracy in the chip area. The maintaining of such matching accuracy and joint accuracy corresponds to three-dimensional alignment in the semiconductor chip to be formed on the wafer, and is extremely difficult to achieve in consideration of the deformation of the wafer in the process and the errors in the units of the stepper (fluctuations in the alignment accuracies and fluctuation in the running characteristic of the wafer stage).
A possible solution to this problem is to align each of the exposure areas on the wafer with the reticle to be subjected to superimposed exposure, by means of an alignment sensor, immediately before each exposure. Although this method can ensure satisfactory alignment, it is associated with a low throughput, as the aligning operation is required for each exposure (each shot) on each area of the wafer.
It is therefore expected, also in the exposure sequence with image composing, to adopt the enhanced wafer global alignment (E. G. A.) method for achieving accurate alignment between the reticle pattern and each exposure area on the wafer, as disclosed in U.S. Pat. No. 4,780,617. In this E. G. A. method, prior to the exposure operation of a wafer, a sample alignment is conducted by measuring the position of marks attached to plural shot areas on the wafer, thereby statistically determining six parameters of the offset (x, y) of the center of the wafer, elongation or contraction (x, y) of the wafer, remaining rotation of the wafer and perpendicularity of the wafer stage (or of arrangement of the shot areas), based on the difference between the design positions and the measured positions of the marks. The stepping of the wafer is conducted with correction of the positions of a second shot to be superimposed, based on the design positions and thus determined parameters. This method is advantageous in improving the throughput because detection and measurement of the marks are no longer necessary, after the measurement of a relatively limited number of marks (marks of 3 to 16 shots) in comparison with the total number of shots on the wafer, and in achieving an alignment accuracy, for all the shot areas, equivalent to or better than that in the alignment method for each shot (die-by-die or site-by-site alignment), since the detection error for each mark is statistically averaged by the sample alignment of a sufficient number of marks.
The introduction of such E. G. A. method in the exposure sequence for a 2nd layer involving image composing realizes optimum alignment of the first reticle (pattern A'), second reticle (pattern B'), third reticle (pattern C') and fourth reticle (pattern D') respectively with areas A, B, C and D in each chip area, but this is not necessarily the best aligning method in consideration of the jointing accuracy among the areas A, B, C and D.